NERVE 75 



the nerve. Death of the nerve, as when it is heated to 47° 

 C, at once stops the transmission of an impulse. 



We may at once dismiss the idea that tlie impulse 

 is due to a mere flow of electricity. Electricity travels 

 along a nerve at about 300 million metres per second, a 

 velocity much higher than that of the nerve impulse. 



Two possibilities remain. The impulse may be of the 

 nature of the molecular vibration, such as occurs in the 

 stethoscope which conducts sound vibration, or it may 

 consist of a series of chemical changes, such as cause the 

 activity of protoplasm generally. 



In considering this matter it must be remembered that 

 the amount of energy evolved in a nerve impulse need not 

 be great. All it has to do is to start the activity of the 

 part to which it goes. Hence, if chemical changes are the 

 basis of the impulse, these may be extremely small in 

 amount and difficult to detect, while at the same time 

 recovery may be extremely rapid. 



As a matter of fact, the evidence of chemical changfes in 

 nerve fibres is slight. No change in reaction, no heat pro- 

 duction, ahd no phenomena of fatigue can be demonstrated. 

 But two bits of evidence point to the existence of chemical 

 changes. Oxygen is required ; and the fact that, if an impulse 

 succeeds in passing through a piece of nerve in which con- 

 duction is decreased by cold or by drugs, it regains its 

 original strength must mean that the impulse is due 

 to a chemical change similar to that which occurs as a 

 spark passes along a trail of gunpowder. The spark may be 

 delayed and reduced to a minimum in a damp part of the 

 trail, but, if it passes this, it regains its original strength 

 when a dry part is reached. 



NERVE CELLS. 



1. Automatic Action? — It has sometimes been supposed 

 that nerve cells originate nerve impulses, that they have an 

 automatic action. The nerve cells which send fibres to 

 muscles may be isolated from the influence of incoming 

 neurons by cutting the posterior roots of the spinal nerves 



